Wireless communication networks have traditionally relied upon macro base stations to support relatively large macro cells. However, in order to increase capacity in wireless communication networks, smaller non-macro base stations are being deployed that support “small cells” such as pico cells, femto cells, etc. These small cells are deployed in addition to existing macro cells to increase network capacity and coverage. In some instances, a single Radio Network Controller (RNC) may support a large number of non-macro base stations throughout a wireless communication network (i.e., include those base stations in its Radio Network Subsystem “RNS”). The term RNS refers to one or more RNCs and the base stations that are supported by the one or more RNCs. A RNS is responsible for the resources and for transmission and reception in a set of cells.
According to 3GPP standards (e.g., ETSI TS 125.427 and TS 125.402), a serving RNC (SRNC) performs initial transport channel synchronization when adding a new radio link for a call, as part of the dedicated radio link setup procedure. Because handover in which a given link is added may be performed quite frequently, such transport channel synchronization may be frequently needed. Without performance of such synchronization, it is possible that packets will be dropped due to arriving at their destination outside of an acceptable reception window. Transport channel synchronization is performed by a given RNC between the RNC and all base stations in the so-called “active set” (i.e., all base stations that a given wireless terminal is simultaneously connected to). In some instances, when adding a new radio link at soft handover, an existing transport channel synchronization, which uses an old timing, can still be used. This is more likely to be true in a network that has small transport delays. However, when adding a radio link that exhibits a larger transport delay, transport channel synchronization may need to be reinitialized.
In Universal Mobile Telecommunications System (UMTS) networks, if a new radio link includes a base station supported by a different RNC, such as a drift RNC (DRNC), the SRNC performs transport channel synchronization by sending a synchronization control frame to the base station of the new leg, receiving a response from the base station, and calculating the round trip time (RTT) between the SRNC and the base station of the new leg.
The interface between a RNC and a base station is known as the “Iub” interface, whereas the interface between two RNCs is known as the “Iur” interface. Handovers between cells with different delays belonging to different RNCs are quite common. Delays on the Iur interface are usually rather short as they typically include a good quality transport connection (e.g., a carrier grade connection). Delays on Iub interfaces, however, can vary considerably because the quality of transport solutions connecting a given base station to its corresponding RNC can vary. A given Iub interface may include a fiber optic, asymmetric digital subscriber line (ADSL), or very-high-bit-rate digital subscriber line (VDSL) connection, for example, and these exhibit varying amounts of delay. Moreover, when a given RNC supports a large number of small cells, those Iub interface can vary to an even greater degree, because it is more likely that a non-carrier grade link will be used for the Iub interface of a small cell. In view of this, the RTT time calculations performed in connection with transport channel synchronization can cause considerable delays when adding call legs.